I just want to be clear that you do need to ftp into your server to install the maps.

Follow the path to the maps folder

Server IP>tf>maps

Upload your map files to that folder

After doing that, you need to go back and edit your maplist.txt and maplist.cfg file located in the TF folder. You MUST edit those files to make your server automatically run the map without having to change it through the rcon. Make sure to accurately name the

Thunder Mountain is not the same as pl_thundermountain

If the map is pl_thundermountain_a3, your maplist.cfg/txt files must reflect pl_thundermountain_a3. If it is not correct, it could cause your server to crash.